1. Field of the Invention
The present invention relates to waveguide structures for use in propagation of electromagnetic waves such as microwaves and millimeter waves, and in particular to electromagnetic-band-gap (EBG) structures for suppressing propagation of electromagnetic waves in specific frequency bands. Specifically, the present invention relates to antennas and printed-circuit boards using waveguide structures as well as electronic devices including printed-circuit boards.
The present application claims priority on Japanese Patent Application No. 2009-41356, the content of which is incorporated herein by reference.
2. Description of the Related Art
Various technologies regarding waveguide structures, printed-circuit boards, and antennas for preventing propagation of electromagnetic waves in specific frequency bands have been developed and disclosed in various documents.    Patent Document 1: U.S. Patent Application Publication, US 2005/019051 A1    Patent Document 2: U.S. Patent Application Publication, US 2005/0205292 A1    Patent Document 3: U.S. Patent Application Publication, US 2007/0176827 A1
Recently, methods for artificially controlling frequency dispersions of electromagnetic waves by use of repetitively aligned conductive patches have been provided. Among those structures, structures having band gaps in frequency dispersions are referred to as EBG structures, which are expectedly applied to filters for suppressing propagation of unwanted noise in printed-circuit boards or substrates of device packages.
Patent Document 1 teaches an EBG structure for reducing noise propagating between parallel plates. The EBG structure includes conductive patches which are aligned in a third layer between parallel plates and which serve as capacitances for one conductive plane of the parallel plates, and shunts (or admittances) for connecting the conductive patches to another conductive plane of the parallel plates, wherein the shunts are repetitively aligned in a one-dimensional manner or a two-dimensional manner along the parallel plates. Due to band gaps occurring in frequency bands in which the shunts serve as inductances in the EBG structure, it is possible to set band gaps by controlling serial-LC resonance frequencies of shunts.
Securing adequate capacitances and inductances in the above EBG structure leads to increasing the areas of conductive patches or increasing the lengths of conductive vias, which in turn makes it difficult to reduce the sizes of structures.
Patent Document 2 teaches a structure in which chip capacitors are mounted on the surface and are connected in parallel between conductive planes and conductive patches. This structure increases capacitances without increasing the areas of conductive patches.
However, Patent Document 2 discloses that using chip capacitors increases the number of parts so as to increase the manufacturing cost.
In the above circumstances, the inventor has recognized that it is necessary to fabricate an EBG structure (or a waveguide structure) and a printed-circuit board with a reduced size, without using chip components, and with low manufacturing cost.
Conventionally, flat panel antennas for microwaves and millimeter waves have been developed, and in particular, small-size and low-frequency antenna structures using the meta-material technology have been developed. This technology is advantageous in reducing sizes of wireless communication devices for use in reception/transmission of electromagnetic waves such as microwaves and millimeter waves.
The relationship between the wave number (or wavelength) and the frequency in electromagnetic waves propagating through dielectric media are referred to as dispersion characteristics of media. Recently, meta-material technologies in which conductive patterns or conductive structures are repetitively aligned so as to artificially control the dispersion characteristics in propagation of electromagnetic waves through structures have been developed and studied in various application fields of engineering.
It is possible to reduce the size of antennas by use of the meta-material technology. Patent Document 3 teaches a small-size antenna structure based on composite right-handed or left-handed (CRLH) principles showing the right-handed property or the left-handed property due to the operating frequency.
The antenna of Patent Document 3 employs a CRLH line structure repetitively aligning a plurality of unit structures including conductive planes, conductive patches disposed in parallel with conductive planes, and conductive vias connected between conductive planes and conductive patches, thus utilizing a length-related resonance of the CRLH line structure in the left-handed frequency range. Conventional media (e.g. right-handed media) suffers from the large size of an antenna structure because the wavelength of electromagnetic waves becomes longer as the frequency becomes lower. In contrast, left-handed media is capable of reducing the size of an antenna structure because the wavelength of electromagnetic waves becomes shorter as the frequency becomes lower.
Patent Document 3 teaches that for the purpose of lowering the frequency range securing the operation of left-handed media, a conductive element is arranged between the conductive plane and the conductive patch so as to increase the capacitance formed between adjacent conductive patches. For the same purpose, a slit is formed in proximity to the connection between the conductive plane and the conductive via so as to form a coplanar line, thus increasing the inductance formed between the conductive plane and the conductive patch.
The inventor has recognized that the left-handed media of Patent Document 3 using the repetitive structure cannot operate as antennas in low frequency range under cutoff frequencies thereof. In short, the antenna structure of Patent Document 3 suffers from the limitation in lowering frequencies. For this reason, it is difficult to design small-size antennas operating in low frequencies based on the conventional technology.